The present invention generally relates to a signal termination circuit which can be used throughout a wide range of frequency bands and which can be applied with a DC biasing voltage.
An electronic circuit for a Gb/s (giga bits/sec.) optical communication system requires a wide frequency characteristic ranging from nearly DC to a frequency substantially in the range of the desired transmission speed. For example, 2.4 Gb/s light communication requires a frequency band of from 10 kHz to 3 GHz.
In a microwave range, circuit blocks are connected to each other through a transmission line. In this case, a signal must be terminated at the input of the circuit block with a wide range of frequency band.
Meanwhile, for many reasons, such as small dimension, high reliability etc. a semiconductor integrated circuit (which will be sometimes referred to merely as the IC, hereinafter) is used. Generally speaking, the IC is mounted in a package. In a GHz frequency band, it is required to locate the signal termination within the package. Further, a biasing voltage must be applied to an input terminal of the IC; the other end of a terminating resistor must be floated with respect to DC components of the signal and be grounded with respect to high frequency components thereof.
A circuit which satisfies the above conditions is shown in FIG. 2. The illustrated circuit includes an input terminal 1 connected to the preceding stage, package pins P1 and P2, an IC 2, an IC package 3, a terminating resistor RL, a DC blocking capacitor Cb, and by-pass capacitors Cp.sub.1 and Cp.sub.2. The terminating characteristic of the circuit at low frequencies is determined by a total of the values of the by-pass capacitors. Generally speaking, a capacitor having an excellent high-frequency characteristic has a small capacitive value. Thus the excellent terminating characteristic throughout a wide range of frequencies is obtained by using a plurality of capacitors. FIG. 3 is a characteristic diagram of the circuit of FIG. 2 showing a relationship between terminating impedance and frequency, in the case where RL=50 .OMEGA., Cp.sub.1 =80 pF and Cp.sub.2 =0.1 .mu.F. It will be observed form FIG. 3 that the characteristic curve has a peak in the vicinity of 300 MHz The peak is due to the resonance which occurs between the parasitic inductances of the capacitors Cp.sub.1 and Cp.sub.2 and the capacitances thereof. With the circuit used in the measurement of FIG. 3, the parasitic inductances of the capacitors Cp.sub.1 and Cp.sub.2 were 0.4 nH and 2.5 nH respectively. Since it is actually impossible to completely eliminate these parasitic inductances, it has been impossible to remove the peak in the terminating-impedance to frequency characteristic of the system of FIG. 2.